This invention relates to a dual type hydraulic circuit for a vehicle brake, more particularly, to one which is capable of maintaining proper distribution of braking power to the front and the rear wheels, not only when both braking fluid circuits are in normal function but even when either one circuit is faulty, i.e., has fallen into a pressure failure by any cause.
It has recently been conventional to divide a vehicle braking system into two mutually independent ones, with the object of enhancing running safety, such that when one circuit should have a failure during a drive the other remaining one may be capable of braking the vehicle.
Another popular attempt is to dispose a proportioning valve (hereinafter dalled a P valve), which is aimed to so control the fluid pressure to the rear wheel cylinder as it rises at a milder gradient than the rising gradient of pressure in the master cylinder when the fluid pressure in the master cylinder rises exceeding a certain predetermined value. It is of course necessary to halt a vehicle, effectively as well as safely (conditions required for this will be called hereinafter an ideal condition), without incurring a so-called locking-up phenomenon (slipping phenomenon between the ground and the rotation-ceased vehicle wheels) to the front and rear wheels.
Combined practice of the dual circuit system and the P valve concept is, however, accompanied by a new problem from a view point of safety. As the P valve mainly aims to control the pressure to the rear wheel cylinder, when both braking systems are normal in function, so as to properly arrange or balance the ratio between the front wheel braking power and the rear wheel braking power, it often contains another disadvantage, that is, in a case of either one circuit having become faulty in raising its pressure due to a conduit breakage or other failure (hereinafter called a one-circuit-failure case), the P valve restricts the rear wheel braking power smaller than it ought to be. When a one-circuit-failure case arises the deceleration rate of a vehicle becomes, as can be easily imagined, smaller than in a case wherein both circuits are in normal (hereinafter called a both-circuits-normal case), so the resultant decrease of forward load transfer brings about a decrease of load reduction on the rear wheels. It means that making the rear wheel braking power larger than that in a both-circuits-normal case will not result in a locking or locking-up of the rear wheels. The P valve inevitably restricts the rear wheel cylinder pressure just like in a both-circuits-normal case, when it is not so required, which means an adverse restriction of the desirable raising of the rear wheel cylinder pressure.
In view of such a situation various attempts and devices have been made so far, providing for a one-circuit-failure case, such as limiting or halting the function of the P valve temporarily or by-passing purposely the pressured fluid without passing through the P valve, in order to fully raise the rear wheel braking power. All of these, however have, disadvantageous in some respects irrespective of effectiveness in some respects. The present invention has been made from such a background.